Various types of wellbore fluids are used in operations related to the development, completion, and production of natural hydrocarbon reservoirs. The operations include fracturing subterranean formations, modifying the permeability of subterranean formations, or sand control. Other applications comprise the placement of a chemical plug to isolate zones or complement an isolating operations. The fluids employed by those operations are known as drilling fluids, completion fluids, work over fluids, packer fluids, fracturing fluids, conformance or permeability control fluids and the like.
Of particular interest with regard to the present inventions are fluids for water control applications, as during the life cycle of a hydrocarbon well, e.g., a well for extracting oil or natural gas from the Earth, the producing well commonly also yields water. In these instances, the amount of water produced from the well tends to increase over time with a concomitant reduction of hydrocarbon production. Frequently, the production of water becomes so profuse that remedial measures have to be taken to decrease the water/hydrocarbon production ratio. As a final consequence of the increasing water production, the well has to be abandoned.
In many cases, a principal component of wellbore service fluids are gelling compositions, usually based on polymers or viscoelastic surfactants.
Viscoelastic surfactant solutions are usually formed by the addition of certain reagents to concentrated solutions of surfactants, which most frequently consist of long-chain quaternary ammonium salts such as cetyltrimethylammonium bromide (CTAB). Common reagents which generate viscoelasticity in the surfactant solutions are salts such sodium salicylate and sodium isocyanate and non-ionic organic molecules such as chloroform. The electrolyte content of surfactant solutions is also an important control on their viscoelastic behaviour.
Further references related to the use of viscoelastic surfactants as wellbore service fluids can be found for example in U.S. Pat. No. 4,695,389, U.S. Pat. No. 4,725,372, and U.S. Pat. No. 5,551,516.
There has been considerable interest in the viscoelastic gels formed from the solutions of certain surfactants when the concentration significantly exceeds the critical micelle concentration. The surfactant molecules aggregate into worm-like micelles which can become highly entangled at these high concentrations to form a network exhibiting elastic behaviour. These surfactant gels are of considerable commercial interest, including application as oil well fracturing fluids.
The viscoelasticity of the surfactant solutions appears invariably to form rapidly on mixing the various components. The resulting high viscosities of the viscoelastic gels can make handling or placement difficult. For example, placement of a uniform surfactant gel in a porous medium is difficult since injection of the gel in the medium can lead to the separation of the surfactant from the solute by a filtration process. Any application of viscoelastic surfactant solutions which requires their transport or placement after their preparation would benefit from a method of controlling their viscosities and gel times.
The gelation of high molecular weight polymers (M.sub.w &gt;10.sup.6 g/mol) has been extensively used in the development of water-based treatment fluids for water control is further described for example by R. D. Sydansk in "Acrylamide-polymer/chromium(III)-carboxylate gels for near wellbore matrix treatments", 7th SPE Symp. Enhanced Oil Recovery, Tulsa, Okla., April 1988, SPE/DoE 20214, or by R. S. Seright in: "placement of gels to modify injection profiles", SPE/DoE Symp. Enhanced Oil Recovery, Tulsa, Okla., April 1994, SPE 27740. Typically for those methods, an aqueous solution of a high molecular weight polymer, such as a polyacrylamide/polyacrylate copolymer (a so-called partially-hydrolysed polyacrylamide), is gelled in situ in a porous formation using a metal crosslinker such as Cr.sup.3+ or small water-soluble organic crosslinkers such as formaldehyde and formaldehyde/phenol. Other water-soluble polymers such as poly(vinyl alcohol), the polysaccharide guar gum and the copolymer poly(vinylpyrrolidone-co-2-acrylamido-2-methyl-1-propanesulphonic acid) which can be crosslinked with a variety of crosslinking agents such as Zr.sup.4+ and boric acid.
A more recent approach is described by A. Keller and K. A. Narh in: "The effect of counterions on the chain conformation of polyelectrolytes, as assessed by extensibility in elongational flow: the influence of multiple valency", J. Polym. Sci.: Part B: Polymer Phys., 32, 1697-1706 (1994). It includes the crosslinking of poly(sodium 4-styrenesulphonate) using Al.sup.3+ ions to form a gel. The concentration of the high molecular weight hydrophilic polymers used to form hydrogels is typically in the range 3-10 g/l.
Copolymers containing polar and non-polar segments are described for example in U.S. Pat. No. 4,776,398. The copolymers are cross-linked in subterranean formations so as to control the permeability of the formation layer before injecting a driving fluid into injector wells.
Furthermore, there have been a number of published studies of the physical gels which are formed by polymer-surfactant interactions. The gelation and viscoelastic behaviour results from specific interactions between the polymer chains and the micelles formed from assembled surfactant monomers. Commonly, the polymers have some fraction of hydrophobic groups on their chains which are associated with (or solubilised in) the surfactant micelle; see for example Sarrazin-Cartalas, A., Iliopoulos, I., Audebert, R. and Olsson, U., "Association and thermal gelation in mixtures of hydrophobically modified polyelectrolytes and nonionic surfactants", Langmuir, 10, 1421-1426 (1994).
Piculell, L., Thuresson, K. and Ericsson, O., in: "surfactant binding and micellisation in polymer solutions and gels: binding isotherms and their consequences", Faraday Discuss., 101, 307-318 (1995).and Loyen, K., Iliopoulos, I., Audebert, R. and Olsson, U., in: "Reversible thermal gelation in polymer/surfactant systems. Control of gelation temperature", Langmuir, 11, 1053-1056 (1995) have given recent accounts of these polymer-surfactant gels. A common example of a polymer-surfactant gel is an aqueous solution containing the polymer hydroxypropylcellulose and the surfactant sodium dodecylsulphate as described for example by Wang, G., Lindell, K. and Olofsson, G., in: "On the thermal gelling of ethyl (hydroxyethyl) cellulose and sodium dodecyl sulfate. Phase behaviour and temperature scanning calorimetric response", Macromolecules, 30, 105-112 (1997).
The object of this present invention is therefore to provide improved compositions for wellbore service fluids based on viscoelastic surfactants. It is a specific object of the invention to enhance the gel strength of such compositions. It is a further specific object of the invention to provide such compositions for water control operations in hydrocarbon wells.